With regard to this issue Shmuel said: With regard to all exempt rulings in the halakhot of Shabbat, although one who performs the action is exempt by Torah law, his action is prohibited by rabbinic law, with the exception of these three for which he is exempt and it is permitted to perform the action.
And another example where he is exempt and it is permitted is: One who drains an abscess containing pus on Shabbat, if he did so to create a permanent opening in it, he is liable. However, if he did so to drain fluid from it, he is exempt.